Hi,

Please refer below article to get previous page url C#

[Solved] How to get the previous page name and URL in ASP.Net using C# and VB.Net

Or you can simply use javascript

HTML

<div>
    <asp:Button Text="Previous" runat="server" OnClientClick="return Back();" />
</div>

Javascript

<script language="javascript" type="text/javascript">
    function Back() {
        history.go(-1);
        return false;
    }
</script>

I hope this will help you out.